1. Field of the Invention
This invention relates to well logging apparatus, and more particularly to a well logging sonde having a shielded collimating window for detecting back-scattered gamma rays from a formation about a borehole.
2. Description of the Prior Art
Nuclear logging tools for transport through a borehole are known which detect photons or gamma rays that have been emitted from a source in the tool and scattered from the formation. If the detected gamma rays have energies from about one hundred thousand electron volts (100 keV) to one million electron volts (1 MeV), the measurement may indicate the electron density and consequently the porosity of the formation.
The attenuation mechanism for gamma rays with energies less than about 100 keV, however, provides an indication of the chemical composition of the formation. Chemical or mineral composition information of the formation is available because these low energy photons are absorbed through the photoelectric effect.
U.S. Pat. No. 3,864,569 to J. Tittman discloses well logging apparatus for identifying the chemical composition of an earth formation and which compensates for mudcake during well logging. Tittman suggests that the formation be irradiated with photons that have energies less than 1 MeV. Tittman recommends that a low energy gamma radiation detector be positioned close to the gamma radiation source primarily to measure borehole parameters, and recommends that a second low energy gamma ray detector be spaced more distantly from the gamma ray source to measure borehole characteristics and the mineral composition of the formation. Signals from the detectors recommended by Tittman's patent are normalized and combined to provide an output that identifies formation lithology and effective mudcake thickness.
The Tittman patent teaches no means for collimating back-scattered gamma rays in the formation in proximity to the near spaced detector. As a result, the gamma rays which are detected by the near spaced detector may come from a wide spatial distribution in the formation. For that reason Tittman's suggested apparatus measures photoelectric effect reactions not only close to the borehole, but may also represent gamma rays which have been Compton scattered from a region deeper into the formation.
U.S. Pat. No. 4,048,495 to Ellis discloses a density logging apparatus in which the near spaced detector does have a collimating window. The collimating window of the Ellis patent provides a shield opaque to gamma rays and preferably formed of tungsten which is provided on the external face of a pad which is pressed against the borehole wall. The near spaced detector of the Ellis patent however, has associated electronics in order to eliminate the influence of the average atomic number of the elements in which the detection diffuses. The electronic count rate of the back-scattered gamma rays detected by the detector is measured in a restricted energy range chosen such that back-scattered gamma rays resulting from Compton scattering is the only consequential form of interaction with the adjacent materials.